FIELD OF THE INVENTION
The present invention concerns a method of marking optical frequencies. This method finds a particular application in a communication network using optical fibers to guide carriers produced by semiconductor laser sources. The carriers are modulated with data to be transmitted and occupy a succession of different frequency channels constituting transmission channels. Some known networks of this kind, known as dispersed source networks, comprise the following components:
A succession of terminals each comprising optical send and receive means for sending and receiving data conveyed by a succession of modulated waves occupying a succession of different frequency channels in a predetermined communication band and resulting from the modulation of a succession of carrier waves of said channels, respectively.
A reference block sending an optical reference wave at a reference frequency.
Optical fibers linking said terminals to each other and to said reference block to transmit said modulated waves and said reference waves.
The send means of each terminal comprise:
a monitored source comprising a semiconductor laser emitting a monitored wave to constitute the carrier wave of one channel, said wave having a monitored frequency constituting a carrier frequency of said channel, PA1 frequency control means for controlling said monitored frequency, PA1 modulation means for modulating said monitored wave and constituting a modulated wave, and PA1 frequency comparator means comparing said reference and monitored frequencies and operating on said frequency control means to lock said monitored frequency relative to said reference frequency. PA1 Journal of lightwave technology, vol. 6 No 0 11 November 1988, pages 1770-1781, DENSELY SPACED FDM COHERENT STAR NETWORK WITH OPTICAL SIGNALS CONFINED TO EQUALLY SPACED FREQUENCIES--B.S. GLANCE, J. STONE, K. J. POLLOCK, P. J. FITZGERALD, C. A. BURRUS, JR., B. L. KASPER, and L. W. STULZ. PA1 FREQUENCY STABILIZATION TECHNIQUES FOR COHERENT LIGHTWAVE SYSTEMS--M. W. MAEDA, SPIE vol. 1175 Coherent Lightwave Communications (1989)--p. 4-11. and the article: EXPERIMENTAL RELATIVE FREQUENCY STABILIZATION OF A SET OF LASERS USING OPTICAL PHASE-LOCKED LOOPS--LEONID G. KASOVSKY and BENITE JENSEN--IEEE PHOTONICS TECHNOLOGY LETTERS. VOL. 2 No 7 JULY 1990 p. 516-518. PA1 a succession of terminals each comprising optical send means and optical receive means for sending and receiving data conveyed by a succession of modulated waves occupying a succession of different frequency channels and resulting from the modulation of a succession of carrier frequencies of said channels, respectively, the set of said channels occupying a predetermined communication band, PA1 a reference unit sending a reference wave having a reference frequency, and PA1 optical fibers linking said terminals to each other and to said reference unit to transmit said modulated waves and said reference wave, PA1 the send means of each of said terminals comprising: PA1 a monitored source consisting of a semiconductor laser emitting a monitored wave to constitute the carrier wave of a channel, said wave having a monitored frequency constituting a carrier frequency of said channel, PA1 frequency control means for controlling said monitored frequency, PA1 modulator means for modulating said monitored wave and constituting a modulated wave, and PA1 frequency comparator means comparing said reference and monitored frequencies and operating on said frequency control means to lock said monitored frequency relative to said reference frequency, PA1 in which network said reference frequency effects at least one scanning half-cycle according to a known law as a function of time and during which said frequency progressively moves from a first end to a second end of a scanning band on a go path and then returns from said second to said first end on a return path, said scanning band being chosen so that said scanning half-cycle produces a go frequency coincidence during said go path and then a return frequency coincidence during said return path, said frequency comparator means of each terminal measuring a marker interval consisting of the time interval between the go and return frequency coincidences of the same scanning half-cycle, said frequency comparator means producing an error signal representing a difference between the marker interval and a set point interval, said error signal being supplied to said frequency control means to lock said monitored frequency to a set point frequency defined by said set point interval.
Other networks of this kind are called grouped source networks and comprise a plurality of sources in the same terminal.
The availability of indium phosphide semiconductor laser sources which are strongly coherent and whose wavelength can be tuned has made it possible to adopt dense spaced multiplexing techniques in implementing these various networks. These techniques are directed to enabling these networks to operate with as small as possible a spacing between adjacent channels, to obtain the maximum benefit from the transmission window of the fibers around the frequency defined by the wavelength 1 550 nm.
The resulting drawback, which constitutes a major obstacle to the development of this type of technique, resides in the need to stabilize the frequency at which these sources emit and to define them with great accuracy. The problem is prevent unwanted progressive drift of the carrier frequency of one channel creating problems of crosstalk with an adjacent channel such as to compromise transmission. Also, the carrier frequency provides the means of identifying the channel and must therefore be defined absolutely. This is why this frequency must be monitored, that is to say measured or at least marked, the source and the carrier wave being therefore also monitored.
The problem is particularly difficult in the case of dispersed source networks because the sources cannot be grouped together at the same geographical site; this is the case with an interactive communication network in particular.
Various solutions have been proposed for this problem: the reference frequency may be a single frequency. It may then be distributed through the same fibers as the modulated waves if this frequency is in the margin of the communication band occupied by the transmission channels. The monitored sending source within a terminal is then stabilized relative to this reference frequency using a fixed or scanning Fabry-Perot interferometer. On this point reference may usefully be had to the article:
In one alternative solution a plurality of reference frequencies can constitute what is known as a "comb". The distribution of a comb of optical frequencies is supported by an ancillary fiber network so as not to disturb the use of the transmission channels. The frequency comb may be produced within the reference block by angular modulation of a coherent reference source. In this case the optical spectrum has a series of equi-distant lateral lines whose spacing is equal to the modulation frequency. Each monitored source can then be locked in frequency or in phase to one of the lateral lines of the reference source after heterodyne or homodyne detection. Reference may usefully be had on this topic to the article:
In the simpler case where the sources are grouped together in the same terminal, which is the case with a distribution network, other techniques have been proposed. One is the "heterodyne spectroscopy" technique described in the article: COHERENT OPTICAL-FIBER SUBSCRIBER LINE--E. J. BACCHUS--R. P. BRAUN--W. EUTIN--E. GROBMANN--H. FOISEL--K. HEIMES--B. STREBEL.--ELECTRONICS LETTERS Dec. 5, 1985 Vol. 21 No 25/26--P. 1203-1205. Another is the so-called "reference impulse" method described in the article: SHIMOSAKA--THG3 Frequency Locking of FDM optical sources using widely tunable DBR LDs--N. SHIMOSAKA--K. KAEDE, S. MURATA--OFC.88/THURSDAY MORNING/168. A common feature of these methods is the use of a so-called "master laser" reference source the optical frequency of which is periodically scanned across the band occupied by the network. The reference wave that it emits is mixed with all the channels and by heterodyne detection this produces a succession of electrical impulses at coincident frequencies.
In the so-called "heterodyne spectroscopy" technique the current value of the master laser control electrical signal at the time of the pulse is compared with its nominal value--value representing the frequency reserved to this channel. Any difference with respect to this nominal value is directly related to the frequency offset and can be used to correct the sending frequency of the monitored source.
The so-called "reference pulse" technique uses a stream of reference pulses produced by detecting the lightwave emitted by the master laser and transmitted by a calibration Fabry-Perot cavity whose free band gap is precisely equal to the frequency spacing to be maintained between the carrier frequencies of adjacent channels. Any time offset between a reference pulse and a respective coincidence pulse indicates drift of the respective monitored frequency and constitutes an error signal that can be used to lock that frequency.
A particular object of the present invention is to enable simple implementation of a communication network with different frequency channels operating securely despite close spacing between adjacent channels. A more general object of the present invention is to define an optical frequency marking method which is both simple and accurate so that using it in a communication network with different frequency channels makes it possible to achieve the previously indicated particular object of the invention, even if the sources to be monitored are divided between geographical locations.